Diet is a modifiable cardiovascular disease (CVD) risk factor and a core component of population-based approaches for CVD prevention. Yet, the exact nature of what constitutes an optimal diet for cardiovascular health is debated. There is a critical need for studies that delineate the biologic sources of well-documented, but poorly understood heterogeneity in diet responsiveness. Gut bacteria are intimately involved in nutrient absorption and metabolism, but until recently we lacked the tools to efficiently characterize the whole microbiome in large-scale studies. New work supports a mechanism for increased CVD risk through gut microbial-dependent metabolism of dietary choline or L-carnitine to trimethylamine N-oxide (TMAO). These findings have received much attention as they may point to interventions through diet or targeted pro/pre- biotics; they have also raised questions about the impact of TMAO precursors on CVD risk. However, the pathway from diet to CVD through microbial production of TMAO has not been studied in population-based samples. We propose to study this pathway in a population-based prospective epidemiologic study: the biracial (black/white) Coronary Artery Risk Development in Young Adults (CARDIA) study of 5,115 individuals followed over 25 years (baseline 1985/86; aged 18-30). My long-term career goal is to build an independent research program dedicated to understanding individual variability in metabolic response to diet in relation to CVD risk. To this end, I seek to integrate my expertise in epidemiology, diet, nutrition, and biostatistical analysis, with substantive trainig in gut microbiome and human metabolism-key biologic components of variability in dietary response-and technical training in bioinformatics and statistical approaches to high-dimensional data analysis. My long-term career goal is to build an externally-funded, collaborative research program that bridges epidemiology with new methodologic and technologic advances related to high-dimensional data and translational science. My scientific aim is to determine the extent to which individual-level metabolic variability plays a role in dietary effects on CVD through gut microbial functions on nutrient absorption and metabolism. My central hypothesis is that dietary risk factors for CVD are partially mediated by gut microbial community composition and function. I have assembled an outstanding team of mentors, with expertise in human metabolism, nutritional biochemistry, and metabolomics (Zeisel); trans-disciplinary research and integration of high-dimensional biologic data (genomics) into epidemiologic studies (Gordon-Larsen); gut microbiome biology and bioinformatics analysis of gut microbiome data (Fodor); statistical analysis of high-dimensional data (Sun); and the design of ancillary case- cohort studies within multi-center cohort studies (Heiss). Career development and research activities will culminate in successfully competing for an R01 to study gut microbial pathways for heterogeneous diet responsiveness, particularly with respect to the cardiovascular system in a large-scale epidemiologic study.